These plies, which are intended to constitute the carcase reinforcement or crown reinforcement plies of the tire, are formed from continuous threads that are approximately mutually parallel and arranged with a given transverse pitch. The threads are embedded in rubber.
The term “thread” is understood to mean any individual or cabled reinforcement thread, which is itself formed by an assembly of individual filaments. The threads may be, without distinction, of textile, composite or metallic nature.
The processes most widely used to manufacture these straight-thread plies consist in depositing a layer of rubber on both sides of the ply of threads using a calendar formed from two rolls capable of pressing the rubber sheets against each other so that the rubber penetrates into the free space between the threads until forming rubber bridges.
Also known are sheathing means in which the ply of threads passes through a coating die upstream of which there is a coating chamber communicating with a feed means capable of delivering a rubbery product under pressure and at a given flow rate. The ply of threads runs from a thread guide placed at the inlet of the device into the coating chamber, where the mixture under pressure penetrates into the free spaces between the threads, and emerges from the device by passing through a sizing die intended to give the semi-finished ply a precise thickness.
Material feed means, such as for example one or more single-screw extruders, feed the coating chamber via one or more channels opening onto the upper part or onto the lower part of the ply of threads.
This technique has, inter alia, the advantage of allowing very regular series of rubber bridges to be formed because of the very good impregnation of the threads due to the high pressure within the coating chamber. Its use is particularly suitable for producing narrow straight-thread plies, similar to strips, and intended for production, by winding said strips onto the blank of a tyre in the process of being built. Another application consists in producing reinforcement plies making a given angle to the circumferential direction, by juxtaposing portions of strips at a given angle.
However, the manufacture of modern tyres requires assemblies having increasingly complex and precise geometries, making it necessary to reconsider the use of the known means.
FIG. 1 shows a straight-thread ply of the type sought, in which the rubber compound A covers the threads F so that the upper back of the threads is located at a distance eup from the upper surface of the ply and the lower back is located at a distance elow from the lower surface of the ply, the total thickness of which is denoted by Etot. It should also be noted that, in this type of ply, the distances eup and elow are not necessarily the same.
To manufacture a straight-thread ply as illustrated in FIG. 1 using a sheathing device, the thicknesses eup and elow are varied by displacing the plane of the ply of threads relative to the upper and lower edges of the die. The total thickness etot is determined by the diameter of the threads, the run speed of the threads through the sheathing device and the pressure within the coating chamber.
This results in a number of difficulties associated with controlling the position of the ply of threads relative to the surface. This is because any variation in pressure between the flow of compound intended to feed the upper part of the ply and the flow of compound intended to feed the lower part of the ply causes the threads to move upwards or downwards.
To give an illustration, a pressure difference of 1 bar between the two flows over a length of 10 mm generates a force of 0.6 N perpendicular to the surface of the ply. It would be possible to increase the tension of the ply so as to prevent them from undergoing too great a displacement in the direction perpendicular to the surface of the ply. However, this technique remains limited in that the threads would then be liable to cut or damage the ply formed by an uncured and still very plastic elastomer.
The differences in pressure between the flows intended to feed material into the coating chamber, at the upper part and the lower part of the ply of threads respectively, are the consequence of a large number of parameters that it is not easy to bring under control. This applies in particular when it is desired to vary the pressure in the coating chamber according to the run speed of the ply of threads so as to maintain a constant total thickness Etot. Other parameters also have an influence, such as the instantaneous temperature of the members in contact with the rubber compound, the localized differences in rheology of the compound, the extrusion speed, the diameter of the cords, the pitch of the cords and finally the shape and the geometry of the flow channels.
This results in a geometric irregularity in the rubber thicknesses on the back of the threads, which is not compatible with the desired precision.